Materials such as nitride semiconductors, including GaN, AlN, and InN, and mixed crystals of such nitride semiconductors, have a wide band gap. Hence, such materials having the wide band gap are used for high-output electronic devices, short wavelength light emitting devices, or the like. For example, GaN, which is an example of the nitride semiconductor, has a band gap of 3.4 eV that is wider than a band gap of Si that is 1.1 eV and also wider than a band gap of GaAs that is 1.4 eV.
An FED (Field Effect Transistor), and particularly an HEMT (High Electron Mobility Transistor) as proposed in Japanese Laid-Open Patent Publication No. 2002-359256, for example, is an example of the high-output electronic device. The HEMT using the nitride semiconductor may be used for a high-output and high-efficiency amplifier, a high-power switching device, or the like. More particularly, in the HEMT that uses AlGaN for an electron supply layer and GaN for an electron transit layer, a piezoelectric polarization or the like may occur in AlGaN due to a distortion caused by a difference between lattice constants of AlGaN and GaN, to thereby generate 2DEG (2-Dimensional Electron Gas) having a high concentration. For this reason, this HEMT may operate at a high voltage, and may be used in a high-efficiency switching device, a high voltage power device for an electric vehicle, or the like.
In some ultra-high-frequency devices using the nitride semiconductor, InAlN having a high spontaneous polarization is used for the electron supply layer in place of AlGaN, in order to obtain a high output from the device. Even a thin InAlN layer may induce the 2DEG having a high concentration, and thus, InAlN is being regarded as a material enabling both high output properties and high frequency properties.
However, in the HEMT that uses InAlN for the electron supply layer, the high spontaneous polarization of InAlN increases an electric field intensity within the electron supply layer. Particularly in the case of an HEMT having a Schottky gate electrode, there is a problem in that a gate leak current increases.
Accordingly, it is desirable to reduce the gate leak current in the HEMT that uses InAlN for the electron supply layer and has the Schottky gate electrode.